The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Internal combustion engines combust an air and fuel mixture within cylinders to produce drive torque. A byproduct of combustion is exhaust gas. The exhaust gas may include various components such as nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC). An exhaust treatment system includes a catalyst that converts the NOx, CO, and HC to carbon dioxide and water.
Conversion efficiency of the catalyst refers to the ability of the catalyst to react with or convert one or more components of the exhaust gas. The conversion efficiency of the catalyst is related to the temperature of the catalyst. The catalyst may operate at a reduced conversion efficiency when the catalyst temperature is less than a threshold temperature. The catalyst efficiency may be increased by increasing the catalyst temperature to be greater than or equal to the threshold temperature.
The catalyst temperature may be increased using various methods. For example only, heat from the exhaust gas exiting the engine may increase the catalyst temperature. The exhaust gas transfers heat to the catalyst via convection, thereby increasing the catalyst temperature. Fueling to the engine may also be adjusted to increase the catalyst temperature. For example only, unburned fuel from the engine may enter the catalyst where the fuel combusts with oxygen in the catalyst to increase the catalyst temperature. Air may be pumped into the exhaust gas and/or catalyst to increase the amount of oxygen in the catalyst.
Hybrid vehicles may include an internal combustion engine and an electric motor. The electric motor may be used more often when vehicle kinetic energy can be recovered by regenerative braking, converted to electric and chemical form, and stored in a battery, from which the motor is driven (e.g. in city driving). The internal combustion engine may be used when wheel braking and opportunities for energy recovery are infrequent, and the engine operates at its greatest efficiency (e.g. in highway driving). In mixed city and highway driving conditions, the electric motor and internal combustion engine may be used together to transmit power to a transmission input shaft, depending on driving conditions and the magnitude of the battery capacity.
Hybrid vehicles may experience long periods of engine off-time during idle and driving scenarios. During the periods of engine off-time, the catalyst temperature may decrease below the threshold temperature. Accordingly, catalyst heating may be required to obtain peak efficiency of the catalyst. Maintaining the catalyst temperature at approximately the threshold temperature during engine-off periods increases the conversion efficiency of the catalyst when the engine is started.